1. Field of the Invention
The present invention relates to a lubricating composition, more specifically, to a lubricating composition which has excellent stability against oxidation, prevents increase of acid value and sludge formation, and also has low corrosivity to non-ferrous metals.
2. Description of the Related Art
Lubricating oil is used in various fields as automobile lubricating oil used for internal combustion engines, driving mechanisms including automatic transmissions, dampers, or power steerings and gears; gear oil or bearing oil for various kinds of machineries and tools; hydraulic oil, which also serves as power transmission fluid on operations such as transmission, control, or damping of power in hydraulic systems of hydraulic apparatuses and devices; metalworking oil used for metalworking including cutting, grinding, or plastic working; and the like. Lubricating grease is also used in the same manner.
These lubricating oil compositions, that is, lubricating oil and grease, inevitably suffer from shortening service life caused by oxidative deterioration with oxygen or NOx, and therefore, they are usually admixed with an antioxidant so as to extend their service life as long as possible.
For example, lubricating oil for internal combustion engines (engine oil) mainly lubricates various sliding parts such as piston rings and cylinder liners, bearings for crankshafts or connecting rods, or valve operating systems involving cams and valve lifters. In addition, engine oil cools inside of engines, diffuses combustion products in it for cleaning and prevents rust and corrosion.
Thus, various kinds of performances are required for engine oil. Moreover, engine oil is recently expected to have still higher performances because internal combustion engines with higher performance and higher output have been developed and operated under more and more harsh conditions. Therefore, in order to satisfy these demands, various kinds of additives such as abrasion inhibitors, metal detergents, ashless dispersants, and antioxidants are admixed in the engine oil.
On the other hand, gas engine heat pump systems and cogeneration systems have been recently developed and commercialized for air-conditioners of houses or buildings. These systems involve gas engines which generally use natural gas, liquefied petroleum gas (LPG) or the like as fuel. However, because maintenance services have increased with popularization of these systems, improvements in maintenance services including simplifying checkout services and prolonging maintenance interval have become a critical issue. There is, thus, a demand for long-life engine oil, which exhibits high resistance against oxidative deterioration and can prolong drain interval.
Incidentally, lubricating oil for gas engine heat pumps, because it experiences a high combustion temperature and for reasons relating to the structure of these equipments, has a problem that it tends to deteriorate rapidly upon contact with NOx which is contained at a high concentration in blow-by gas. Therefore, investigations have been pursued to prolong drain interval of the oil by improving its oxidation stability, particularly oxidation stability against NOx. As methods of improving oxidation stability of lubricating oil, there have been used so far, for example, a method of selecting a base oil or an additive (which is a base material for lubricating oil) excellent in oxidation resistance; a method of selecting an appropriate additive which suppresses effectively oxidative deterioration of the base material; and the like.
As an example of long-life gas engine heat pump engine oil which is excellent in oxidation resistance against NOx and particularly reduces burden of maintenance services, there is disclosed a lubricating composition which contains poly(alkenylsuccinimide) and/or a boron derivative thereof, specific diarylamines, and specific hindered phenols in a given proportion, respectively as described, for example, in Japanese Patent Application Laid-Open No. 7-126681.
Nonetheless, the aforementioned conventional technology still does not provide satisfactory stability against oxidation and the actual situation is that an aimed drain interval has not been attained yet.
As for hydraulic oil, it serves as power transmission fluid used for operations such as transmission, control, or damping of power in hydraulic systems of hydraulic power equipments or devices. The hydraulic oil works also to lubricate sliding parts.
Nowadays, hydraulic power equipments have been progressed along the line of miniaturization and increase in output, being accompanied by still higher operating pressures (for example, an operating pressure of 30 MPa or higher is now used in place of a conventional pressure of 14 MPa to 20 MPa) while the volumes of their oil tanks have become smaller. For this reason, still higher thermal load is applied to actuating shafts, and this causes the problems of early deterioration, sludge formation, abnormal odor, chattering of cylinders, mal-operation, and others.
Conventionally, zinc alkyldithiophosphate (ZnDTP), which works as an antioxidant and a lubricant, has been used for hydraulic oil. However, such hydraulic oil still has the problems of sludge formation, mal-operation caused by this sludge, abnormal odor, and others, because ZnDTP is thermally decomposed at hot sections where the temperature is locally elevated by compression heat of gas bubbles with an increase in pressure.
As an example of hydraulic oil which effectively prevents early deterioration and sludge formation in high pressure operation, has a long service life, and also shows stable performance without chattering of cylinders, there has been proposed a hydraulic oil composition which comprises an aminic antioxidant, a phenolic antioxidant, a phosphate ester, and an aliphatic amide and/or a polyol ester in a specific ratio respectively added to a base oil having a % CA of 5 or less as described, for example, in Japanese Patent Application Laid-Open No. 9-111277.
This hydraulic oil composition is, although its performances are rather improved as compared with conventional oil compositions, still unsatisfactory in performances in long-term service.
Until now, as a part of the study towards extending service life of lubricating oil, development of more effective antioxidants and technology of combinations thereof have been examined. In such technology, ZnDTP has been conventionally used in many cases. ZnDTP not only works as an antioxidant but also has a large effect on abrasion prevention. ZnDTP works as a corrosion inhibitor as well, and has been widely used, particularly for engine oil.
However, recent tightening of the automobile emission regulation has led to installation of exhaust gas cleaning equipments in automobile engines. Lead-free gasoline and low-phosphorus engine oil have been introduced so as to prevent poisoning of the catalyst used in the exhaust gas cleaning equipment. Following this event, use of ZnDTP also has begun to be limited. Therefore, it has become essential to develop phosphorous-free antioxidants.
Conventionally, it is well known that a synergetic effect can be obtained by using a sulfur-based antioxidant and a phenolic antioxidant in combination. As the sulfur-based antioxidant, however, an antioxidant having a monosulfide structure has been mainly used, and there is a problem of increase of acid value caused by hydrolysis. Further, sulfur-based compounds having a trisulfide or higher polysulfide structure have a problem of having a larger corrosivity to non-ferrous metals.